In a radio system such as a point to point system, for example, a parabolic antenna and an array antenna are used.
As shown in FIG. 1, a general array antenna is constructed of a plurality of antenna elements provided on a printed board and a power supply circuit for supplying electric power to these antenna elements. Circles in the drawing designate the antenna elements, and solid lines to connect the respective antenna elements designate lines constructing the power supply circuit (microstrip lines). As shown in the drawing, the antenna elements are arrayed in the shape of a grid on the printed board.
In the array antenna constructed in this way, unnecessary radiation (grading lobe) will be generated in a direction different from a direction in which radiation is maximum (main lobe direction). When this radiation is large, side lobe characteristics will be degraded.
In the drawing, it is assumed that an X-Y plane including an X axis and a Y axis is parallel to the plane of paper and that a Z axis is in a direction vertical to the X-Y plane. It is assumed that this Z axis is a main lobe direction. All of the antenna elements and the lines of the power supply circuits are provided on the printed board on the X-Y plane. Moreover, the antenna elements are arranged in a row in the X axis direction and a plurality of rows are arranged side by side in the Y axis direction. It is assumed that this X axis direction is an array direction in which the antenna elements are arrayed.
Let's consider a case in which, as shown in FIG. 2, the antenna elements are arrayed at element intervals d on the plane of the board. In the drawing, it is assumed that an X-Z plane including the X axis and the Z axis is parallel to the plane of the paper and that the Y axis is in a direction vertical to the X-Z plane.
Here, in a case where the element interval d is longer than a half wavelength of a radiation wave, an unnecessary radiation beam will be generated in addition to a main beam in the main lobe direction (Z axis direction). Specifically, unnecessary radiation will be generated in a θn direction to satisfy the following equation (1).sin θn=sin θo+nλ/d  (1)where in the equation (1), θo is the direction of the main beam (main lobe), θn is the direction of unnecessary radiation, n is a natural number, and d is the element interval (interval of a wave source of unnecessary radiation).
For example, in a case where n=1, θo=0, and d=1.4×λ, θn can be calculated by the equation (2).θn=arc sin(1/1.4)=45(deg)  (2)That is, in this case, unnecessary radiation will be generated in a direction of 45 degrees when viewed from the main lobe direction.
In FIG. 2, a case will be shown as an example where an antenna element itself is a wave source of the unnecessary radiation. However, also a branch point of the microstrip line can be a wave source of the unnecessary radiation. Arrows in FIG. 1 show the wave sources of the unnecessary radiation. In the drawing, the unnecessary radiation is generated at a branch point between the adjacent antenna elements. The direction of the arrow of unnecessary radiation designates the direction of an electric field and is the same direction of a polarized wave of the element of the antenna (omitted in the drawing).
The side lobe characteristics of the array antenna will be degraded by the generation of unnecessary radiation.
Specifically, FIG. 3 is a view to show a radiation pattern of an array antenna in which the unnecessary radiation of the microstrip line is reduced. FIG. 4 is a view to show a radiation pattern of an array antenna which is affected by unnecessary radiation of the microstrip line. In FIG. 3 and FIG. 4, a vertical axis indicates a gain (dB) and a horizontal axis indicates an angle made by a main lobe direction and the direction of a radiation wave.
As shown in FIG. 3, the maximum value of a side lobe level in a case in which unnecessary radiation of the microstrip wiring is not generated is −31.8 dB, whereas the maximum value of a side lobe level in a case in which unnecessary radiation of the microstrip line is generated is −21.6 dB as shown in FIG. 4. In this way, unnecessary radiation of the microstrip line has a significant effect on the side lobe characteristics of the array antenna.
In general, in a radio system, an antenna having excellent side lobe characteristics is required in many cases so as to suppress unnecessary radiation to the surroundings.
Thus, in a general array antenna, a power supply circuit may be provided on a board different from a printed board on which antenna elements are provided. According to this construction, unnecessary radiation generated at the branch point of the microstrip line does not have an effect on a plane on which the antenna elements are provided and hence the side low characteristics of the array antenna can be improved. Furthermore, in an array antenna described in a patent document 1, unnecessary radiation from the power supply circuit is reduced by providing a shield plate on a circuit on which a power supply circuit is provided.